Diamond Material on Blender Render engine?
As suggested by user277143 I created a Gradient Texture using Photoshop and then overlapped the mesh to give an internal refraction effect. The effect is very subtle but is there.
On the screen shot you can see that I added (perhaps too many) flat meshes as light sources, additional to a Sun and Hemi lights, in order to create some additional reflection. (The Material reflective is 0.25)
I think it looks marvelous, just like I wanted it. :)



Edit: user277143 also suggested using Shade smooth (Ctrl+F > Alt+H), which I did, and the results on the borders are marvelous, albeit for some reason it changes the reflections on the flat areas. ;)